Filtration is a widely used solid-liquid separation process. Depending upon the application, a wide variety of filtration media ranging from simple screens to complex ultrafiltration membranes are available. A well-known and persistent problem with filtration processes is particle (or solute) polarization and the ultimate fouling of the filtration media.
As insoluble particles or solutes are rejected by the filter media, they accumulate at the surface, where insoluble particles form cakes, and at high concentrations, solutes precipitate to form gel layers. Permeate flow resistance due to the cake or gel layer is often much greater than that of the filtration media. As filtration continues, these layers increase in thickness and become compacted, resulting in ever lower transmembrane flow. When resistance reaches a level making filtration impractical, the filter membrane or screen is said to be fouled.
Therefore, filter cleaning inevitably becomes an all too frequent step for filtration processing. Conventional cleaning techniques, which can range from back-flushing and automated scraping to manual scraping and membrane replacement, are time consuming and generally require a shut-down of the filtration process. Where the filtration functions as a step in an otherwise continuous process, two or more filters are typically arranged in parallel to allow periodic cleaning by rotating service. Such process shut downs and extra equipment of course increase the overall cost of filtration processing.
Efforts at overcoming the fouling problem are reflected in various techniques developed for removing the cake or gel layers without interrupting the filtration process. For example, the cross-flow filtration technique has found widespread use. With the cross-flow technique, the filtration membranes are not only penetrated from the unfiltered or concentrate side to the filtrate or permeate side, but on the concentrate side, a strong cross-flow is generated along the membrane surface. This flow of concentrate (or parent fluid) along the membrane surface reduces the overall build-up of the gel layer and therefore, slows membrane fouling.
One alternate configuration for cross-flow filtration involves the use of rotating elements in a stationary vessel. U.S. Pat. No. 4,911,847 to Shmidt et al. illustrates a filtration device employing a membrane mounted on an inner body which rotates within a stationary outer body.
A widely used approach is periodic back-washing without a complete shut-down, i.e., during the filtration, the filtration media is briefly subjected to pressure from the filtrate side, against the direction of permeate flow. Filtration systems employing finer membranes do not respond well to such back-washing, however.
Techniques including vibrational energy have also been developed. U.S. Pat. No. 4,158,629 to Sawyer discloses a filter device having a mechanically resonating housing that transmits wave energy through the flowing concentrate liquid. Unfortunately, this technique requires expensive sound isolation equipment and yields only limited benefits because the ultrasonic energy quickly dissipates in the fast flowing concentrate.
This and other conventional techniques suffer from one or more additional drawbacks as follows: high energy costs, rapid fouling, high equipment cost, frequent membrane damage and maintenance of mechanical elements.
Thus, there continues to be a need for an improved filtration system that can more effectively and efficiently prevent membrane fouling. Specifically, it would be desirable to provide the advantages of back-flush cleaning without interrupting the filtration process. Furthermore, there would be great advantage to a filtration system with ultrasonic energy enhancement suitable for use in a cross-flow configuration.